1. Field of the Invention
The invention relates to a technology for determining whether a malfunction occurs in a cooling apparatus for an internal combustion engine, and particularly to a technology for accurately determining whether a thermostat is malfunctioning when a cooling medium, which flows through an inside of an internal combustion engine, is circulated by an electric pump.
2. Description of the Related Art
A cooling apparatus is provided for an internal combustion engine, and the cooling apparatus cools constituent components heated by heat generated by combustion. The cooling apparatus includes a thermostat. When the temperature of the cooling medium is lower than a threshold value during warming-up, the coolant is circulated so that the coolant bypasses the radiator, using the thermostat. After the temperature of the cooling medium reaches the threshold value and the warming-up is completed, the cooling medium is circulated via the radiator, using the thermostat. Thus, the internal combustion engine is quickly warmed-up, and the internal combustion engine is maintained at an appropriate temperature. However, if the thermostat is malfunctioning, for example, the opening degree of a valve in the thermostat is fixed, the cooling medium is circulated via the radiator even during the warming-up. As a result, it is not possible to quickly warm-up the internal combustion engine. Therefore, it is necessary to accurately determine whether a malfunction occurs in the cooling apparatus.
In view of the above, for example, Japanese Patent Application Publication No. 2005-207393 (JP-A-2005-207393) describes a malfunction determination apparatus for an engine cooling system, which more accurately determines whether a malfunction relating to adjustment of cooling capacity of the engine cooling system occurs. The malfunction determination apparatus determines whether a malfunction occurs in the engine cooling system in which coolant that has flowed out from a water jacket of an internal combustion engine is cooled, and then, the coolant flows into the water jacket. The malfunction determination apparatus includes determination means for determining that that a malfunction occurs when an amount of change, per unit time, in the temperature of the coolant flowing into the water jacket from the engine cooling system is equal to or larger than a predetermined determination value.
In the malfunction determination apparatus described in the above-described publication, it is possible to accurately determine that a malfunction relating to adjustment of the cooling capacity of the engine cooling system occurs.
However, when the cooling medium is circulated in a medium passage included in a cooling apparatus using an electric pump, there is a possibility of making an erroneous determination as to whether a thermostat is malfunctioning, for the following reason. In the case where the electric pump is operated when the temperature of the coolant of the internal combustion engine is equal to or higher than a predetermined value, the cooling medium that has stayed in the water jacket and heated by the operation of the internal combustion engine starts to flow due to the operation of the electric pump, and accordingly, a value detected by a sensor that detects the temperature of the cooling medium temporarily increases. Therefore, when the thermostat is malfunctioning, it may be erroneously determined that the thermostat is normally functioning.
Even in the malfunction determination apparatus described in the above-described publication, there is a possibility of making an erroneous determination relating to the thermostat due to a sharp change in an outlet temperature caused by the operation of the electric pump during warming-up, and this problem cannot be solved.